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The Sun's atmosphere is filled with ultrasound-like waves that may help solve decades-old mysteries about the Sun and space weather, according to Southwest Research Institute scientists who found the waves in data from NASA's TRACE spacecraft. Astrophysical Journal Letters reports on the findings in its December 10 issue.

Dubbed "solar ultrasound," the waves are approximately 300 times deeper
than the deepest pitch audible to the human ear, at a frequency of 100
millihertz (10-second period).

"At 10-second period, these waves qualify as ultrasound because individual
atoms on the Sun experience only a few collisions during the brief passage
of each wave, just as with ultrasound here on Earth," says Dr. Craig
DeForest, a senior research scientist in the SwRI Space Science and
Engineering Division. DeForest found the signature in TRACE data collected
in January 2003.

The waves are most likely created by the sudden collapse of magnetically
induced electric currents (magnetic reconnection) or by lower frequency
sound waves that crash like ocean waves as they make their way up from the
surface of the Sun. Both of the sources are likely candidates for the
source of the solar atmosphere's mysterious extra heat, making the new
waves a valuable tool for exploring a decades-old mystery.

At up to 100,000 deg C (180,000 deg F), the chromosphere, or middle solar
atmosphere, is nearly 20 times hotter than the 6,000 deg C (11,000 deg F) surface
of the Sun. The solar corona, at 1,000,000 deg C (1,800,000 deg F), is about 10
times hotter still, or 200 times hotter than the surface of the Sun.
Although scientists have been studying the process for more than 50 years,
the reason for this difference in temperature remains elusive.

"By examining these waves more closely, we should be able to discern the
source of energy release in the solar atmosphere, just like you can tell by
listening whether the car is running in a dark garage," says DeForest. "In
both cases, something is releasing energy into the environment, and that
release has a recognizable sonic signature."

The Sun is filled with lower-pitched waves, at about 3 mHz (5-minute
period), that are used to probe the solar interior and even to make images
of the far side of the Sun. The solar ultrasound is too high pitched to be
directly related to these more well-known "photospheric oscillations."

Sound waves cannot travel through interplanetary space, so they are
detected remotely as small fluctuations in the brightness of solar
ultraviolet emissions. The TRACE spacecraft, built by Lockheed-Martin for
NASA's Explorer program, is an ultraviolet telescope in orbit around
Earth. The solar ultrasound is at the limit of detectability by TRACE --
so faint that individual waves cannot be resolved. Instead, DeForest
sleuthed for patterns in the background noise of the telescope.

"Each individual wave train has an amplitude of about one-tenth of the
smallest brightness value that TRACE can see," he says. "But when we
average many images together in the right way, a pattern emerges that we
can recognize as the signature of trapped waves." The pattern emerges
through three-dimensional Fourier analysis, a mathematical technique that
isolates individual types of motion from the morass of activity above the
solar surface.

Although the waves appear faint, they are quite energetic. "These ripples
seem to be carrying about 1 kilowatt of power per square meter on the
surface of the Sun," says DeForest. "That is similar to the sonic energy
you might find coming out of the speakers at a rock concert. Very loud."

"The discovery of coherent waves at such high frequencies in the upper
solar atmosphere challenges our understanding of the magnetic structures
in the quiet Sun," says Dr. Joseph Gurman, TRACE mission scientist at
NASA's Goddard Space Flight Center. "This work proves that we need new
tools to understand the propagation of energy into and out of this part of
the solar atmosphere where much of the activity that can affect life here
on Earth originates."

The open data policy of the TRACE mission boosts scientific productivity by
allowing researchers and the general public to download solar data
collected by the spacecraft. This allows individuals to re-use the data
for purposes beyond those of the original mission.

Future instruments will be able to better detect the waves. While TRACE is
a simple telescope that can detect only changes in brightness, the waves
are likely to have a much stronger Doppler signature. One problem remains,
however; Earth's ozone layer is opaque to the ultraviolet light used to see
and "hear" the solar ultrasound, making the light difficult to detect from
the ground. SwRI scientists are designing rocket- and balloon-borne
instruments to observe from above the bulk of Earth's atmosphere to catch a
better glimpse of the wave spectrum for probing the solar atmosphere.

The NASA Sun-Earth Connection Guest Investigator Program funded the study
that revealed the wave signatures.

SwRI is an independent, nonprofit, applied research and development
organization based in San Antonio, Texas, with more than 2,800 employees
and an annual research volume of more than $339 million.

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